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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
sendmsg — send a message on a socket using a message structure
SYNOPSIS
#include <sys/socket.h>
ssize_t sendmsg(int socket, const struct msghdr *message, int flags);
DESCRIPTION
The sendmsg() function shall send a message through a connection-mode or connectionless-mode socket. If
the socket is a connectionless-mode socket, the message shall be sent to the address specified by msghdr
if no pre-specified peer address has been set. If a peer address has been pre-specified, either the
message shall be sent to the address specified in msghdr (overriding the pre-specified peer address), or
the function shall return -1 and set errno to [EISCONN]. If the socket is connection-mode, the
destination address in msghdr shall be ignored.
The sendmsg() function takes the following arguments:
socket Specifies the socket file descriptor.
message Points to a msghdr structure, containing both the destination address and the buffers for the
outgoing message. The length and format of the address depend on the address family of the
socket. The msg_flags member is ignored.
flags Specifies the type of message transmission. The application may specify 0 or the following
flag:
MSG_EOR Terminates a record (if supported by the protocol).
MSG_OOB Sends out-of-band data on sockets that support out-of-bound data. The
significance and semantics of out-of-band data are protocol-specific.
MSG_NOSIGNAL Requests not to send the SIGPIPE signal if an attempt to send is made on a
stream-oriented socket that is no longer connected. The [EPIPE] error shall
still be returned.
The msg_iov and msg_iovlen fields of message specify zero or more buffers containing the data to be sent.
msg_iov points to an array of iovec structures; msg_iovlen shall be set to the dimension of this array.
In each iovec structure, the iov_base field specifies a storage area and the iov_len field gives its size
in bytes. Some of these sizes can be zero. The data from each storage area indicated by msg_iov is sent
in turn.
Successful completion of a call to sendmsg() does not guarantee delivery of the message. A return value
of -1 indicates only locally-detected errors.
If space is not available at the sending socket to hold the message to be transmitted and the socket file
descriptor does not have O_NONBLOCK set, the sendmsg() function shall block until space is available. If
space is not available at the sending socket to hold the message to be transmitted and the socket file
descriptor does have O_NONBLOCK set, the sendmsg() function shall fail.
If the socket protocol supports broadcast and the specified address is a broadcast address for the socket
protocol, sendmsg() shall fail if the SO_BROADCAST option is not set for the socket.
The socket in use may require the process to have appropriate privileges to use the sendmsg() function.
RETURN VALUE
Upon successful completion, sendmsg() shall return the number of bytes sent. Otherwise, -1 shall be
returned and errno set to indicate the error.
ERRORS
The sendmsg() function shall fail if:
EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and the requested operation would block.
EAFNOSUPPORT
Addresses in the specified address family cannot be used with this socket.
EBADF The socket argument is not a valid file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR A signal interrupted sendmsg() before any data was transmitted.
EINVAL The sum of the iov_len values overflows an ssize_t.
EMSGSIZE
The message is too large to be sent all at once (as the socket requires), or the msg_iovlen member
of the msghdr structure pointed to by message is less than or equal to 0 or is greater than
{IOV_MAX}.
ENOTCONN
The socket is connection-mode but is not connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The socket argument is associated with a socket that does not support one or more of the values
set in flags.
EPIPE The socket is shut down for writing, or the socket is connection-mode and is no longer connected.
In the latter case, and if the socket is of type SOCK_STREAM or SOCK_SEQPACKET and the
MSG_NOSIGNAL flag is not set, the SIGPIPE signal is generated to the calling thread.
If the address family of the socket is AF_UNIX, then sendmsg() shall fail if:
EIO An I/O error occurred while reading from or writing to the file system.
ELOOP A loop exists in symbolic links encountered during resolution of the pathname in the socket
address.
ENAMETOOLONG
The length of a component of a pathname is longer than {NAME_MAX}.
ENOENT A component of the pathname does not name an existing file or the path name is an empty string.
ENOTDIR
A component of the path prefix of the pathname in the socket address names an existing file that
is neither a directory nor a symbolic link to a directory, or the pathname in the socket address
contains at least one non-<slash> character and ends with one or more trailing <slash> characters
and the last pathname component names an existing file that is neither a directory nor a symbolic
link to a directory.
The sendmsg() function may fail if:
EACCES Search permission is denied for a component of the path prefix; or write access to the named
socket is denied.
EDESTADDRREQ
The socket is not connection-mode and does not have its peer address set, and no destination
address was specified.
EHOSTUNREACH
The destination host cannot be reached (probably because the host is down or a remote router
cannot reach it).
EIO An I/O error occurred while reading from or writing to the file system.
EISCONN
A destination address was specified and the socket is already connected.
ENETDOWN
The local network interface used to reach the destination is down.
ENETUNREACH
No route to the network is present.
ENOBUFS
Insufficient resources were available in the system to perform the operation.
ENOMEM Insufficient memory was available to fulfill the request.
If the address family of the socket is AF_UNIX, then sendmsg() may fail if:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the pathname in the
socket address.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an
intermediate result with a length that exceeds {PATH_MAX}.
The following sections are informative.
EXAMPLES
Done.
APPLICATION USAGE
The select() and poll() functions can be used to determine when it is possible to send more data.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
getsockopt(), poll(), pselect(), recv(), recvfrom(), recvmsg(), send(), sendto(), setsockopt(),
shutdown(), socket()
The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SENDMSG(3POSIX)